1. Technical field
The solution according to one or more embodiments of the present disclosure generally relates to the field of electronics. More particularly, this solution relates to switching systems for igniting combustion engines.
2. Description of the Related Art
Nowadays, in almost every combustion engine (for example, in the automotive field) the ignition of a corresponding combustion is usually provoked by an ignition system of the electronic type. Typically, the ignition system comprises a switching device (for example, an insulated gate bipolar transistor, or IGBT), which controls the ignition sparks of spark plugs of the combustion engine. For this purpose, the IGBT is coupled with a primary winding of a transformer; the transformer has one or more secondary windings, each of which is coupled with a respective spark plug. During each cycle of the combustion engine, the IGBT is firstly turned on by applying a suitable voltage to its gate terminal. As a result, the primary winding is charged with a current having a substantially linear pattern. To create the ignition spark, the IGBT is turned off causing an abrupt cut of the corresponding current. Consequently, an extra-voltage develops across the primary winding (proportional to the change rate of the current of the IGBT); this extra-voltage (properly clamped by a high voltage zener Z, in order to avoid an eventual breakdown of the IGBT) is reflected to each secondary winding multiplied by a turns ratio (i.e., the ratio between the number of turns of a conduction wire in the secondary winding and the number of turns of a conduction wire in the primary winding) of the transformer. Therefore, a very high voltage (in the order of thousands of Volts) is set across each spark plug causing the firing of the ignition spark.
However, the current through the IGBT also changes when it turns on (according to a duration of a turn-on transient period of the IGBT). This causes a corresponding extra-voltage across the primary winding, which results in an overshoot voltage across the secondary winding that may cause an undesired ignition spark. Such undesired ignition spark may provoke an earlier ignition of the combustion engine, thereby lowering the efficiency or even causing serious engine damages, since the anomalous spark could occur in a wrong point of the combustion engine cycle.
In order to solve this problem, it is known in the art to control the IGBT in such a way to obtain a so-called soft turn-on thereof, wherein the IGBT is turned on gradually. For this purpose, it is possible to apply a relatively small direct current to the gate terminal of the IGBT; this current charges corresponding stray capacitors of the IGBT, to increase the gate voltage slowly until the IGBT turns on. In this way, a change rate of the current across the IGBT is greatly reduced (thereby avoiding any undesired ignition spark).
However, some operating parameters of the IGBT (such as its threshold voltage) are strictly related to environment conditions (such as an external temperature). Therefore, the soft turn-on of the IGBT may lose efficiency whenever the value of its operating parameters varies with respect to the expected values according to which the soft turn-on is designed. Moreover, values of the operative parameters may randomly differ from nominal values due to non-ideality inherent to the manufacturing process of the IGBT (e.g., manufacturing tolerances), from now on indicated as manufacturing process spreads for sake of conciseness. The values of the operative parameters may also change because of an aging of the IGBT. Thereby, such variations in the operative parameters cause a permanent efficiency loss of its soft turn-on. In addition, it is also possible that, for the above-mentioned reasons, the same operating parameters become inadequate for switching the IGBT on, with the consequence that the IGBT would remain always off, preventing the combustion engine from operating.
Another technique known in the art for reducing the overshoots on the secondary winding calls for providing a voltage limiter device between each secondary winding and the respective spark plug.
However, such technique is inherently expensive since for each spark plug of the engine a corresponding voltage limiter device should be provided.